1. Field of the Invention
The present invention relates to a flash circuit, and more particularly to a flash circuit which automatically stops charging a main capacitor at a set charge voltage.
2. Background Arts
When the subject brightness is so low that a proper exposure would not be provided without any artificial illumination, a flash device is often used to project light toward the subject synchronously with the shutter release. Since compact cameras and lens-fitted photo film units have an inexpensive lens system of a relative large f-number, most of them are provided with a built-in flash device. For flash photography, it is necessary to charge the main capacitor up to the set voltage prior to the shutter release. The conventional flash devices start charging in response to an actuation of a flash charge switch.
A flash circuit has recently been known, for example from JPA 7-122389, wherein once a flash charge switch is turned on the main capacitor continues to be charged even after the flash charge switch is turned off. The flash circuit stops charging while the main capacitor is at the set charge voltage. Hereinafter, this type of flash device will be referred to as an autostop flash circuit.
An example of autostop flash circuit is shown in FIG. 13, which has fundamentally the same configuration as that disclosed in JPA 7-122389, except some minor differeces. In the flash circuit of FIG. 13, when a flash charge switch 80 is turned on, an oscillation transistor 81 is activated and starts oscillating due to positive feedback of an oscillation transformer 82. The oscillation causes an increase in a primary current that flows through a primary coil 82a, i.e. a collector current that flows to the collector of the oscillation transistor 81. As a result, an electromotive force induces a current through a secondary coil 82b, and the current charges a main capacitor 84 through a rectifying diode 83.
Since the collector current flows through the oscillation transistor 81, a latching transistor 85 is turned on. Thereafter when the increment of the primary current goes down, a back electromotive force is generated in the secondary coil 82b, the current fed back from the oscillation transformer 82 to the oscillation transistor 81, i.e. base current of the oscillation transistor 81, begins to decrease. However, a voltage from a battery 86 is applied to the base of the oscillation transistor 81 through the latching transistor 85 as being in the ON state, the oscillation transistor 81 is not completely turned off. Therefore, the primary current starts flowing again, thereby the oscillation transistor 81 continues to oscillate and charge the main capacitor 84.
A Zener diode 87 with a Zener voltage of 300V is provided for starting conducting a Zener current when the main capacitor 84 is charged up to a set voltage of 300V. Accordingly, when the main capacitor 84 is charged up to the set voltage a stopping transistor 88 is turned on by a base current that is applied to the base of the stopping transistor 88 due to the Zener current. When the stopping transistor 88 is turned on, the emitter and the base of the oscillation transistor 81 are connected to each other, so that the oscillation transistor 81 is completely turned off, and thus the latching transistor 85 is turned off. In this way, the oscillation stops to stop charging the main capacitor 84.
In the above autostop flash circuit, since the charge voltage of the main capacitor is applied to the Zener diode to conduct the Zener current for activating the stopping transistor when the charge voltage reaches the set value, the Zener diode must have a high Zener voltage, e.g. 300V, in correspondence with the set charge voltage. As the Zener diode with high Zener voltage is expensive, it raises the cost of the flash circuit. In addition, the conventional autostop flash circuit requires a lot of space for mounting various elements as above which are necessary for the automatic continuation and stopping of charging.